1. Field of the Invention
The present invention relates generally to linear-beam microwave tubes and, more particularly, to collectors for linear-beam microwave tubes. For descriptive purposes, a traveling-wave tube is used as an exemplary linear-beam microwave tube.
2. Description of the Related Art
An exemplary linear-beam microwave tube in the form of a traveling-wave tube (TWT) 20 is illustrated in FIG. 1. The elements of the TWT 20 are generally coaxially arranged along a TWT axis 21. They include an electron gun 22, a microwave structure in the form of a slow-wave structure 24 (embodiments of which are shown in FIGS. 2A and 2B), a beam-focusing structure 26 which surrounds the slow-wave structure 24, a signal input port 28 and a signal output port 30 which are coupled to opposite ends of the slow-wave structure 24 and a collector 32. A housing 34 typically extends from the collector 32 to protect the other TWT elements.
In operation, a beam of electrons is launched from the electron gun 22 into the slow-wave structure 24 and is guided through that structure by the beam-focusing structure 26. A microwave input signal 36 is inserted at the input port 28 and moves along the slow-wave structure to the signal output port 30. The slow-wave structure 24 causes the axial velocity of microwave signal propagation to approximate the velocity of the electron beam.
As a result, the beam's electrons are velocity-modulated into bunches which interact with the microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal; the signal is amplified and is coupled from the signal output port 30 as an amplified signal 38. After their passage through the slow-wave structure 24, the beam's electrons are collected in the collector 32.
The beam-focusing structure 26 is typically configured to develop an axial magnetic field. A first configuration includes a series of annular, coaxially arranged permanent magnets 40 which are separated by pole pieces 41. The magnets 40 are arranged so that the opposed faces of adjacent magnets have opposite magnetic polarities. This beam-focusing structure is comparatively light weight and is generally referred to as a periodic permanent magnet (PPM) structure. In TWTs in which output power is more important than size and weight, a second beam-focusing configuration often replaces the PPM structure with a solenoid 42 (partially shown adjacent the input port 28) which carries a current supplied by a solenoid power supply (not shown).
As shown in FIGS. 2A and 2B, TWT slow-wave structures generally receive an electron beam 52 from the electron gun (22 in FIG. 1) into an axially-repetitive structure. A first exemplary slow-wave structure is the helix 43 shown in FIG. 2A. A second exemplary slow-wave structure is the coupled-cavity circuit 44 shown in FIG. 2B. The coupled-cavity circuit includes annular webs 46 which are axially spaced to form cavities 48. Each of the webs 46 forms a coupling hole 50 which couples a pair of adjacent cavities. The helix 43 is especially suited for broad-band applications while the coupled-cavity circuit is especially suited for high-power applications.
In another conventional TWT configuration, an oscillator is formed by replacing the output port 30 with a microwave load. Random, thermally generated noise interacts with the electron beam on the slow-wave structure 24 to generate a microwave signal. Energy is transferred to this signal as it moves along the slow-wave structure. This oscillator signal generally travels in an opposite direction to that of the electron beam (i.e., the TWT functions as a backward-wave oscillator) so that the oscillator signal is coupled from the port 28.
TWTs are capable of amplifying and generating microwave signals over a considerable frequency range (e.g., 1-20 GHz). They can generate high output powers (e.g., &gt;10 megawatts) and achieve large signal gains (e.g., 60 dB) over broad bandwidths (e.g., &gt;10%).
The electron gun 22, the helix 43 and the collector 32 are again shown in the schematic of FIG. 3 which illustrates details of the TWT 20 of FIG. 1 (for clarity of illustration, only a simple representation of the helix 43 is depicted). The electron gun 22 has a cathode 56 and an anode 58 and the collector 32 has a first annular stage 60, a second annular stage 62 and a third stage 64. Because the third stage 64 generally has a cup-like or bucket-like form, it is sometimes referred to as the "bucket" or "bucket stage".
The helix 43 and a body 70 of the TWT are at ground potential. The cathode 56 is connected to the negative (-) lead of a cathode power supply 74 that generates a voltage V.sub.cath and has its positive lead (+) connected to ground. The negative lead (-) of an anode power supply 76 is referenced to the cathode 56 and a positive lead (+) is coupled to the anode 58. This positive cathode-anode voltage establishes an accelerating electric field across an acceleration region 78 between the cathode 56 and the anode 58. Electrons are emitted by the cathode 56 and accelerated across the acceleration region 78 to form an electron beam 52. The beam 52 is further modified by the electric field established by the potential difference between the anode 56 and the grounded helix 43.
The total acceleration voltage of the electron beam 52 is the cathode-to-helix voltage difference (ignoring the small voltage difference that exists between the beam axis and the helix as a result of the negative charge in the electron beam). Since the helix 43 is at ground potential, the beam acceleration voltage is just the absolute value of the cathode voltage V.sub.cath.
When an electron, with mass me and initially at rest, is accelerated by electrostatic fields through a potential difference of V, its resultant kinetic energy is given by 0.5 m.sub.e v.sup.2 =eV, in which v is the electron velocity and e is the electronic charge. The kinetic energy divided by the electronic charge e is therefore equivalent to a voltage. Conventionally, an electron that has been accelerated through a voltage difference of V is said to have acquired a kinetic energy of V volts.
The electron beam 52 travels through the slow-wave structure 43 and exchanges energy with a microwave signal which travels along the slow-wave structure 43 from an input port 28 to an output port 30. Only a portion of the kinetic energy of the electron beam 52 is lost in this energy exchange. Most of the kinetic energy remains in the electron beam 52 as it enters the collector 32. A significant part of this kinetic energy can be recovered by decelerating the electrons before they are collected at the collector walls.
Because of their negative charge, the electrons of the electron beam 52 form a negative "space charge" which would radially disperse the electron beam 52 in the absence of any external restraint. Accordingly, the beam-focusing structure applies an axially-directed magnetic field which restrains the radial divergence of electrons by causing them to spiral about the beam.
However, the electron beam 52 is no longer under this radial restraint when it enters the collector 32 and, consequently, it begins to radially disperse. In addition, the interaction between the electron beam 52 and the microwave signal on the slow-wave structure 43 causes the beam's electrons to have a "velocity spread" as they enter the collector 32, i.e., the electrons have a range of velocities and kinetic energies. When the beam travels a given axial distance in the collector 32, the slower electrons are exposed to the divergent force of the electron beam's space charge for a greater time than are faster electrons. Therefore, at a given axial plane and away from the region near the axis where the radial force is small, the energy of electrons within the collector 32 generally decreases with increased radial distance.
Electron deceleration is achieved by application of negative biasing voltages to the collector (by way of feedthroughs 87 in FIG. 1). The potential of the collector is "depressed" from that of the TWT body 70 (i.e., made negative relative to the body 70). The kinetic energy recovery is further enhanced by using a multistage collector, e.g., the collector 32, in which each successive stage is further depressed from the body potential of V.sub.B. For example, if the first collector stage 60 has a potential V.sub.1, the second collector stage 62 a potential V.sub.2 and the third collector stage 64 a potential of V.sub.3, these potentials are related by V.sub.B =0&gt;V.sub.1 &gt;V.sub.2 &gt;V.sub.3 as indicated in FIG. 3.
The voltage V.sub.1 on the first stage 60 is typically depressed to a value that will maximally decelerate but still collect the slowest electrons 80 in the electron beam 52. Sometimes, if there are only a few low-energy electrons in the entering beam 52, higher overall efficiency can be obtained if the first stage 60 is depressed even more. The slowest electrons then have insufficient energy to enter the region of the first stage 60; these electrons are forced to turn around and return to the body potential, either on the region 70 of FIG. 3 or on the grounded helix 43. Higher overall efficiency results if, with greater depression, the increase in energy recovered from the more energetic electrons exceeds the energy lost by collecting the slowest electrons at ground potential.
Successive collector stages 62 and 64 are operated with increasingly depressed voltages to decelerate and collect successively faster electrons in the electron beam 52, e.g., intermediate-energy electrons 82 are collected by collector stage 62 and high-energy electrons 84 are collected by collector stage 64. This process of improving TWT efficiency by decelerating and collecting successively faster electrons with successively greater depression on successive collector stages is generally referred to as "velocity sorting" (velocity sorting is described in many TWT references, e.g., see Hansen, James, W, et al., TWT/TWTA Handbook, Hughes Aircraft Company, 1993, Torrance, Calif., pp. 58-59).
The efficiency enhancement realized by velocity sorting of the electron beam 52 can be further understood with reference to current flows through a collector power supply 88 whose negative lead (-) is coupled to the negative lead (-) of the cathode power supply 74 and whose positive leads (+) are coupled to the collector stages 60, 62 and 64. If the potential of the collector 32 were the same as the collector body 70, the total collector electron current I.sub.coll would flow back to the cathode power supply 74 as indicated by the current 90 in FIG. 3, and the input power to the TWT 20 would substantially be the product of the cathode voltage V.sub.cath and the collector current I.sub.coll.
In contrast, the currents of the multistage collector 32 flow through the collector power supply 88. The input power associated with each collector stage is the product of that stage's current and its associated voltage in the collector power supply 88. Because the voltages V.sub.1, V.sub.2 and V.sub.3 of the collector power supply 88 are a fraction (e.g., in the range of 30-70%) of the voltage of the cathode power supply 74, the TWT input power is effectively decreased. Efficiencies of TWTs with multistage collectors are typically in the range of 25-60%, with higher efficiency generally associated with narrower bandwidth.
If the voltage on a collector stage is depressed too far, electrons will be repelled from the stage rather than being collected by it. Axially-located electrons are especially vulnerable to this rejection. These repelled electrons flow to less-depressed stages or to the TWT body or they may reenter the energy exchange area of the slow-wave structure. In addition, secondary electrons are generated when the electron beam's electrons strike the surfaces of the collector stages. If not properly controlled by the electric fields within the collector, these secondary electrons may also flow to the TWT body 70 or they may reenter the energy exchange area.
Electron rejection to less-depressed stages or to the TWT body reduces the TWT's efficiency. Electron flow to the slow-wave structure interferes with the energy exchange process. This interference often degrades TWT performance by adding gain and phase ripple components over the TWT's frequency bandwidth.
Various collector structures have been introduced to enhance the flow of primary electrons to more-depressed collector stages and to block the flow of secondary electrons from the collector. These structures include transverse vanes, axial probes, external magnets and slanted collector apertures. However, implementing vanes is often mechanically or thermally difficult, probes require the generation and application of additional bias voltages, external magnets require additional test time to properly locate and attach them and slanted apertures are only effective for small apertures.